Electromagnetic actuating means for wire printers



Nov. 16, 1965 R. s. BRADSHAW 3,217,640

ELECTROMAGNETIC ACTUATING MEANS FOR WIRE PRINTERS 4 Sheets-Sheet 1 Filed April 30, 1963 INVENTOR. ROBERT S. BRADSHAW AGENT GOG-:25

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ELECTROMAGNETIC ACTUATING MEANS FOR WIRE PRINTERS 4 Sheets-Sheet 2- Filed April 50, 1965 INVENTOR. ROBERT S BRADSHAW BY W K l Mz AGENT R. s. BRADSHAW 3,217,640

ELECTROMAGNETIC ACTUATING MEANS FOR WIRE PRINTERS Fig.6C

INVENTOR.

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v w COMBINED X FORCES OF I CR LP POSITION RI.

United States Patent G 3,217,649 ELECTRQMAGNETIQ ACTUATING MEANS FOR WIRE PRINTERS Robert S. Bradshaw, Broomail, Pa, assignor to Burroughs Corporation, Detroit, Mich, a corporation of Michigan Filed Apr. 3@, 1963, Ser. No. 276,793 11 Claims. (Cl. 101-93) This invention relates generally to high speed serial wire printing and in particular, to a novel means for actuating print wires of a wire printer.

Serial printing by means of wire printers is well known and is illustrated for example, in US. Patent No. 2,730,- 040, R. B. Johnson, Ian. 10, 1956. Selected wires are first displaced from the normal plane of the wire ends in a matrix and thereafter the matrix including the wire ends is forced against an inked ribbon whereby the selected wires print the desired character. A primary advantage of matrix pin printing lies in the inherent speed advantage which is obtained by forming any desired character from a single print element rather than the requirement of selecting and positioning one of many type elements. The speed of printing has heretofore been limited by the time required for reliable setup mechanisms to select the character defined by the displaced wires in the matrix, plus the time to thereafter force the matrix against the inked ribbon.

Accordingly, the main object of the invention is to provide a fast, simple and reliable wire printing device in which printing takes place without requiring the additional time for subsequent movement of the matrix against the printing ribbon.

It is another object of this invention therefore, to provide in a printer, an improved print wire actuating device which avoids one or more of the disadvantages of the prior art arrangements and which has greater speed.

In accordance with the invention, the printer employs a polar magnet type of transducer for driving the individual print wires at high cyclic rates. The wires or pins are assembled to converge adjacent a platen in a 9 x matrix, and to fan out at their ends so that each pin is connected to the end of a pivotally mounted armature. A plurality of such armatures are contained in a basket-like unit assembly, and are supported in a constant magnetic field flux which extends between a pair of field pole pieces. The armatures in each unit assembly are encircled by individual armature coils and are separated one from another by armature pole pieces positioned in line with the field pole pieces. When a current pulse is applied to an armature coil with a certain polarity, the armature becomes magnetized and the print end thereof moves forward toward the field pole of opposite polarity to print. The circuitry employed causes a current pulse of opposite polarity to be applied through the armature coil to thereafter rotate the armature in an opposite direction to thus return the pin to its starting position.

For a better understanding of the invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings and its scope will be pointed out in the appended claims.

In the drawings:

FIG. 1 is a perspective view of a wire printer in accordance with a preferred embodiment of the invention;

FIG. 2 is a view showing means for constraining the ends of the print wires in matrix form adjacent the platen;

FIG. 3 is a view partly schematic, taken along the line 3-3 of FIG. 1;

FIG. 4 is a partial sectional view showing the sliding connection between the print wire and its associated armature;

Patented Nov. 16, 1965 FIG. 5 is a view similar to FIG. 4 showing the print wire striking the paper;

FIG. 6A illustrates a circuit diagram employed to energize the armature coils of the printer;

FIG. 6B and FIG. 6C are timing diagrams showing time relationships between the input pulse to the energizing circuit and the current through the armature coils;

FIG. 7A is a schematic illustration, showing the reac tion of the armature to the magnetic field flux;

FIG. 7B is a curve illustrating the magnitude of the field force F at the end of the armature for its motion between opposed poles;

FIG. 8A is a schematic illustration showing the reaction of the armature when its coil is energized and operates in a uniform magnetic field flux;

FIG. 8B is a curve illustrating the magnitude of the force during travel of the armature between opposed armature pole faces when subjected to the flux field as in FIG. 8A;

FIG. 9 shows the result of the combined forces to which the armature is subjected.

In one illustrative embodiment of the invention as shown in FIGS. 1 and 2, a polar magnet pin printer 20 employs an assembly of forty-five wires 22, which converge and enter apertures 25 of a 9 x 5 matrix plate 24, supported adjacent the platen 26. The assembly of print wires fan out at their opposite ends and each wire connects with an armature 28. For convenience, five such armatures are mounted in a basket-like unit assembly 30, there being nine such units, five located on one side of the printing center line and four on the other. The unit assemblies 30 stacked one upon another, are disposed between and attached to, pairs of vertical field pole pieces 32 and 34. The pieces of each pair are interconnected at their lower ends by a core 36 by means of bolts 37, and the core is encircled by a field coil 38 which may be energized to provide a constant magnetic field flux between the pole pieces. A carrier 40 suitably supporting the pairs of pole pieces and unit assemblies therebetween is movable along an axis parallel to the platen 26 as indicated by the double arrow XX, to enable the print head matrix 24, to be positioned at successive character positions along the platen. As shown in FIG. 2, the matrix plate 24 is secured to carrier 40 by screws 41. Characters formed in the matrix by the print wires, in a manner hereinafter to be described, may be printed on the fly while the print matrix traverses the printing line of the platen at a constant velocity.

With reference now to FIG. 3 the description of the armatures 28 in each unit assembly is shown. Each armature 28, five such armatures being shown in each assembly, is pivoted near its center by means of a pivot 42, secured on bottom mounting plate 44. The latter mounting plate in turn has mounted thereto a front end armature pole piece 46, shown in the lower portion of FIG. 3, and a rear armature pole piece 48 secured thereto as by screws 46a at its outer ends. The top cover plate 45 has been broken away for clarity to show the end armture pole pieces 46, 48 which are generally T-shaped in outline form and suitably fastened at their shank portions 47, 49 respectively, to the vertical field pole pieces 32, 34 as by bolts 50. The transverse portions of the opposed T-shaped end pole pieces each include a recess as at 51 to thereby provide projecting pole faces 53, 55 in the front and rear end armature poles 46, 48 respectively. A plurality of intermediate armature pole pieces 5? of generally H-shaped configuration are secured to the lower mounting plate 44 by suitable means and disposed in line with said end poles and between adjacent armatures 28 in the unit. Each of the intermediate pole pieces 59 include pole faces 60, 61, 62 and 63 which are in alignment with the end pole faces 53, 55 of the end pole pieces 46, 48. The latter pieces, armatures 28, and the intermediate armature pole pieces 59, may be formed of magnetic materials having magnetic properties of high permeability, low hysteresis and low eddy currents. It is desirable however, that the base mounting plate 44 and the top cover plates 45 be formed of non-magnetic properties such as aluminum. In addition, spacer shims 23a of non-magnetic material are affixed to opposite surfaces of armature 28 for engagement with the faces of the aforementioned pole pieces for purposes as will be later described.

Still referring to FIG. 3, each of the armatures 28 is shown encircled by means of an armature coil 65 shown extending on each side of the pivot 42, and includes taps as at 66, 67. The circuit for energizing each such armature coil may be of the type which allows a single driver to supply a positive pulse to advance the armature and the print wire connected thereto as will be later described. The direction travelled by the magnetic flux (p designated south and north respectively, from the field poles 32, 34, through the series of armature poles 59 is indicated by the arrows When a current pulse of a certain polarity is applied to the armature coil 65 the armature becomes magnetized with the print wire ends carrying wire 22 becoming north polarity and the opposite south polarity. The attraction of opposite magnetic polarities in the armature 28 and armature pole pieces 46, 59 produces a counterclockwise rotation of the armature 28 which moves the print wire 22 forward to print. A second current pulse of opposite polarity is then applied to the armature coil 65 in order to rotate the armature 28 clockwise and return the print wire 22 to its starting position. When current is removed from the armature coil 65 the armature 28 remains against pole face 61 of armature pole piece 59 and against pole face 55 of end pole piece 46 in a stable condition due to the attraction of the field magnetism Considering now the relationship of armature 28, FIG. 3, to print wire 22, reference is now had to FIGS. 4 and 5 respectively. The connection between the armature 28 and the rear end of print wire 22 is by means of a sliding fit, the latter rear end of the wire being received in transverse aperture 29 of armature 28. As the armature is rotated as previously described, from position A FIG. 4, to position B, FIG. 5, the left face 28L of the armature drives the print pin forward by contact with shoulder 23 on the pin. A compression spring 70 encircling the rear extremity of the print Wire is interposed between the right face 28R of the armature and a retaining washer 71 received in groove 72 at the rear tip end of the print wire to retain face 28L of the armature in contact with shoulder 23. When the rotational motion of armature 28 is stopped by the armature pole faces 53, 55, 60 to 63 inclusive, the inertia of the pin continues to carry the same forward until it contacts the ribbon 73 and paper 74 wrapped around platen 26. During the latter impact motion the spring 70 is compressed as shown in FIG. 5. After printing impact the shoulder 23 of the print pin is restored against the armature face 28L by the compression spring. At this time, the pin is clear of contact with the ribbon and paper.

Now referring to FIG. 6A, the driver circuitry previously referred to is shown connected to armature coil 65 as at its two input terminals marked 66, 67. One of the terminals 66 is connected to a capacitor C1 and the other terminal 67 is connected to one polarity of a source of direct current v. Disposed in parallel with capacitor C1 and armature coil 65, is a resistor R1, having one end connected to the DC. source. The other end of resistor R1 along with capacitor C1 are both connected to an electrical switching device, in this example, transistor Q1. Upon receiving an input pulse, the transistor acting as a switching device in a well known manner, connects the circuit to the other polarity of the source of direct current represented by the electrical ground symbol. With the electrical circuit completed, current will flow through the two parallel paths consisting of one path R1 and the other path capacitor C1 and coil 65. Current in the latter path passes through coil 65 and charges up capacitor C1. FIG. 6B shows the time of the input pulse as being from T to T and FIG. 6C shows the amplitude of the current passing through the armature during this time interval T -T Current in the armature coil causes the armature to be magnetized in the direction north to south as indicated in the lowermost armature 28, FIG. 3. This causes the armature to rotate in a counter-clockwise direction to push the print pin forward.

In FIG. 6B at time T the transistor switch is turned off and opens the electrical circuit. The charged capacitor C1 then discharges through resistor R1 and the armature coil. Current flow through the armature coil as shown in FIG. 6C is now in the opposite direction causing the armature to be magnetized in the opposite direction and results in a clockwise rotation of the armature to restore the print pin to its initial position. This movement occurs between times T and T as shown in FIG. 6C.

Referring to FIGS. 7A to 9 inclusive, it may be observed in the schematic diagrams that the force developed by the magnet armature 28 results from a combination of two effects, one being the attraction of the magnetic field (p FIG. 7A, and the other being the force developed by the armature when its encircling armature coil is energized and is acting in the magnetic field, FIG. 8A. The encircling armature coil 65, FIG. 3, is not shown in FIG. 8A for purposes of clarity. The description hereinafter is simplified and does not take into account side effects relating to material magnetization curves, magnetic saturation and fiux leakage etc.

With reference now to FIG. 7A, the armature 28 is illustrated with its lower print wire driving end as being adjacent the left pole face 61 of armature 59, while its upper portion is adjacent face 55 of pole piece 46. The magnetic flux field is shown extending between pole faces 53-61 and 5563. The flux is also illustrated schematically as passing longitudinally through the armature 28 in a direction from pole face 55 to face 61. The force F, developed by the armature is shown plotted in FIG. 7B for its motion between the left pole (L.P.) 61 and the right pole (R.P.) 53. As the curve indicates, the force F is large and negative when in contact with the left pole 61, the force is zero when the armature is in the center, and is large and positive at the right pole 53.

FIG. 8A shows the armature when the field coil 65, not shown, is energized to produce armature flux (p in the uniform field flux As shown by the curve in FIG. 8B, the force F is positive throughout its travel from the left pole 61 to the right pole 53. The magnitude of the force is proportional to the product (b rth and if either quantity goes to zero the force will also be zero.

In FIG. 9 the results of the combined forces of the armature and the field is shown, and is obtained by adding the curve shown in FIG. 7B, to the curve shown in FIG. 813 to obtain the resultant curve C In the region near the left pole 61 it is found that the force changes from a negative to a positive value. The significance of this is seen to be that armature 28 will not produce a force for motion towards the right pole 53 until it has been moved away from the left pole a distance which is slightly greater than the point at which the curve crosses the horizontal axis. This constitutes the starting force and it is a function not only of the magnetic parameters of the device but of the distance of the armature from the pole face as well. Also shown under the curve in FIG. 9, is the shaded area starting at a position beyond where it crosses the horizontal axis and ending at the identical position removed from the right pole face 53 as indicated in that the distance X is equal to X The shaded area S thus represents the available work output of the device which can be used to provide energy for the print pin for impact with the ribbon and paper. Since the armature 28 must be returned in the same manner that it was advanced through its forward or printing stroke, the starting force at the right pole face must also be sufficient to overcome static friction. It is for this reason that the work area boundaries are equal distances from the pole faces, as indicated by the left boundary VV and the right boundary WW. In the apparatus of the present embodiment of the invention, these boundary positions are achieved by using the previ ously referred to non-magnetic shims 28a afiixed to the armature 28 which limit the travel of the armature at the poles faces 53, 61 and 55, 63.

While there has been described what is at present considered to be the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention and it is therefore aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. In a wire printing device, a frame, a pair of magnetic pole pieces spaced from each other on said frame to provide a magnetic field flux between a pair of opposed faces thereof, a plurality of armature means spaced from each other and each mounted for pivotal movement on said frame within said field flux and in a plane extending between said opposed faces, a plurality of armature coil means each associated with one of said armature means, a plurality of print wire means each secured to one of said armature means on one side of its pivot and so mounted for movement that when it moves at least a portion thereof will move in a direction substantially normal to the longitudinal axis of the associated armature means, each said print wire means being mounted for impact engagement with a print medium, control signal means coupled to said plurality of armature coil means and selectively operable to provide control signals in a first direction through selected ones of said armature coil means to rotate their associated armature means in a first direction to impel their print wire means toward said print medium and after a predetermined time interval to provide control signals in the reverse direction through said selected ones of said armature coil means to rotate their associated armature means in a second direction to move their print wire means away from said print medium, and means associated with and including each of said armature means providing a low reluctance path for said magnetic field flux.

2. In a wire printing device, a frame, a pair of magnetic pole pieces spaced from each other on said frame to provide a magnetic field flux between opposed faces thereof, a plurality of spaced apart armature means each mounted for pivotal movement on said frame within said magnetic field flux and in a plane extending between said opposed faces, a plurality of armature coil means each associated with one of said armature means, a print wire means secured to each said armature means on one side of its pivot and so mounted for movement that when it moves at least a portion thereof will move in a direction substantially normal to the longitudinal axis of the associated armature means, each said print wire means being mounted for impact engagement with a printing medium, current control circuit means connected to said plurality of armature coil means and including a single driver for each said armature coil means, said drivers being selectively operable to supply first current pulses to their associated armature coil means to advance their associated armature means and their print wire means and thereafter to automatically supply second current pulses to their associated armature coil means to return their associated armature means and their print wire means, and means associated with and including each of said arma- 6 ture means providing a low reluctance path for said magnetic field flux.

3. In a wire printing device, a frame, permanent magnet means on said frame and having opposed faces to provide a magnetic field flux therebetween, a plurality of spaced apart armature means arranged in a plane and each mounted for pivotal movement on said frame between said opposed faces and within said magnetic field flux, a plurality of armature coil means each associated with one of said armature means, a print wire means secured to each said armature means on one side of its pivot and so mounted for movement that when it moves at least a portion thereof will move in a direction substantially normal to the longitudinal axis of the associated armature means, each said print wire means being mounted for impact engagement with a print medium, electrical means which includes a power source coupled to said plurality of armature coil means and which is selectively operable for applying first current pulses through selected ones of said armature coil means to move their associated armature means in one direction to cause their associated print wire means to print and for applying subsequent current pulses through said selected ones of said armature coil means to move their associated armature means and print wire means in an opposite direction, and means associated with each of said armature means providing a low reluctance path for said magnetic field flux.

4. In a wire printing device as set forth in claim 3 wherein said electrical means includes capacitor means in series with each said armature coil means, and resistive means disposed in parallel with each said capacitor means and each said armature coil means, and wherein said first current pulses cause the capacitor means which are in series with said selected ones of said armature coil means to be charged, and the discharges of said charged capacitor means cause said subsequent current pulses.

5. In a wire printing device, a frame, magnetic means on said frame including a pair of opposed faces to provide a magnetic field flux therebetween, a plurality of spaced apart armature means each mounted for advance and return movements on said frame Within said magnetic field flux between said opposed faces, a plurality of armature pole means each mounted on said frame between adjacent ones of said spaced apart armature means, said plurality of armature pole means and said plurality of armature means providing a low reluctance path for said magnetic field flux, a print wire means mounted to each said armature means for movement in such manner that when it moves at least a portion thereof will move in a direction substantially normal to the longitudinal axis of the associated armature means, a coil means associated with each said armature means, and circuit means selectively operable for energizing selected ones of said coil means in alternate directions to advance and thereafter to return their associated armature means between said opposed faces.

6. In a wire printer having a platen, a frame, a pair of magnetic pole pieces spaced from each other on said frame and adapted to provide a magnetic field flux therebetween, a planar support plate on said frame extending between said magnetic pole pieces, a plurality of spaced apart armature means each mounted for pivotal movement on one surface of said support plate and within said field flux between said pole pieces, a plurality of coil means each encircling one of said armature means, a plurality of print wire means each connected to one of said armature means on one side of its pivot and so mounted for movement that when it moves at least a portion thereof will move in a direction substantially normal to the longitudinal axis of the associated armature means, each said print wire means being movable by its associated armature means between print and non-print positions, a plurality of armature pole pieces mounted to said support plate each between adjacent ones of said armature means, shim means of non-magnetic material secured to opposite surfaces of each one of said armature means to space the same from said armature pole pieces and said magnetic pole pieces when their associated print wire means are positioned in print and non-print positions, and circuit means coupled to said plurality of coil means and selectively operable to receive first electrical signals to energize selected ones of said coil means to thereby move their associated armature means to cause their print wire means to print, and to thereafter automatically provide second electrical signals to energize said selected ones of said coil means to cause withdrawal of their associated print wire means to their non-print positions.

7. In a wire printer having a platen, a frame, magnet means on said frame including a pair of field pole pieces provided with opposed faces and adapted to provide a magnetic field flux therebetween, a plurality of planar supports on said frame each extending between said opposed faces of said field pole pieces, a plurality of spaced apart armature means mounted on one surface of each said planar support, each said armature means being mounted for pivotal movement, a print wire means connected to each said armature means on one side of its pivot and so mounted for movement that when it moves at least a portion thereof will move in a direction substan tially normal to the longitudinal axis of the associated armature means, each said print wire means being movable by its associated armature means between its print and non-print positions, a coil encircling each said arma ture means, a plurality of armature pole pieces each mounted on said frame between adjacent ones of said armature means, means associated with each said armature means to space the same from adjacent ones of said armature pole pieces and from said magnetic field pole pieces when its associated print wire means is alternately positioned in its print and non-print positions, circuit means coupled to each said coil to energize the same to thereby move its associated armature means to cause its associated print wire means to print, and means to thereafter withdraw said associated armature means to move its associated print wire means to its non-print position.

8. In a wire printer having a platen, a frame, a permanent magnet on said frame provided with opposed faces and adapted to provide a magnetic field flux therebetween, a planar support on said frame extending between said opposed faces, a plurality of spaced apart armature means each mounted for pivotal movement adjacent one surface of said planar support, a print wire means connected to each said armature means on one side of its pivot and so mounted for movement that when it moves at least a portion thereof will move in a direction substantially normal to the longitudinal axis of the associated armature means, each said print wire means being movable by its associated armature means between its print and non-print positions, a coil encircling each said armature means, a plurality of armature pole pieces each fixedly mounted on one surface of said planar support and between adjacent ones of said armature means, means associated with said armature means to space said armature pole pieces from said opposed faces of said permanent magnet and from adjacent ones of said armature means when said print wire means are alternately positioned in their print and non-print positions, and circuit means coupled to each said coil to energize the same to thereby move its associated armature means to cause its associated print wire means to print and to thereafter withdraw said associated armature means to move its associated print wire means to its non-print position.

9. In a wire printer having a platen and a matrix, a frame, means on said frame providing a continuous magnetic field flux, first and second groups of armature unit assemblies spaced from each other on said frame and disposed within said field flux, each said unit assembly including a planar support and a plurality of armature pole pieces mounted in spaced relation one to another thereon, a plurality of armatures each pivotally mounted for movement between adjacent armature pole pieces, a plurality of print wire means each attached to an extremity of an armature on one side of its pivot and so mounted for movement that when it moves at least a portion thereof will move in a direction substantially normal to the longitudinal axis of the associated armature, each said print wire means containing a print wire which terminates at one end in the matrix, said print wires of said first and second groups of armature unit assemblies being disposed in side by side relation, means mounted on each said armature to prevent contact between the latter and the armature pole pieces adjacent thereto, armature coil means encircling each said armature, and current control circuit means coupled to each said armature coil means for applying a current through the latter in a first direction to rotate its associated armature in a first direction thereby to impel the print wire attached thereto toward the platen to print, and for thereafter applying a current therethrough in a reverse direction to move its associated armature and print wire away from said platen.

10. In a wire printing device having a platen, a frame, means on said frame providing a constant magnetic field flux, a plurality of armatures each mounted on said frame for pivotal movement in two directions within said field flux, stop means on said frame for limiting movement of each said armature in both said directions and providing with said armatures a low reluctance path for said magnetic field flux, a plurality of print wires each arranged on one side of the pivot of an associated one of said armatures and movable by said associated armature in a direction substantially normal to the longitudinal axis of said associated armature and mounted for sliding movement relative to said associated armature, a coil encircling each said armature, and current control circuit means coupled to said coils and selectively operable for applying a first current through selected ones thereof in a first direction to rotate their associated armatures in a first direction to said stop means, to thereby move their associated print wires forward toward said platen, whereby the inertia of said associated print Wires causes the same to slide relative to their associated armatures to strike said platen, said circuit means being adapted for thereafter automatically applying a current in the reverse direction through said selected coils to cause said associated armatures to move in a second direction to said stop means to move their associated print wires away from said platen.

11. In a wire printing device having a platen, a frame, means on said frame providing a constant magnetic field flux, an armature mounted on said frame for forward and reverse rotational movement within said field flux, a print wire movable by and mounted for sliding movement relative to said armature in a direction substantially normal to the longitudinal axis of the armature, said print wire including an enlarged shoulder portion by means of which it is moved by said armature, coil means associated with said armature, current control circuit means coupled to said armature coil means for applying a current therethrough in a first direction to rotate said armature in a first direction to thereby move said wire forward toward said platen whereby the inertia of said print wire causes the same to slide relative to the armature to stroke said platen, and thereafter for applying a current in the reverse direction through said coil means to cause said armature to move the print wire in an opposite direction, and spring means interposed between said armature and said print wire to impart a sliding motion to said print wire in a direction opposite to that caused by said inertia.

(References on following page) 9 References Cited by the Examiner 2,524,127 10/ 1950 UNITED STATES PATENTS g; 4/1875 EdiSOIl 317137 X 2 35 7 4 5 5 Chapman 1/1909 N1C1101SOI1 317--15O Q 10 5 4 10/1963 3/1920 Cummings 20093.32 9/1938 Loop 197-1 Johnson 101-93 Johnson et a1. 197-1 X Vonglahn 101-93 Fisher 317-150 X Dirks 10193 Preisinger 101-93 WILLIAM B. PENN, Primary Examiner. 

1. IN A WIRE PRINTING DEVICE, A FRAME, A PAIR OF MAGNETIC POLE PIECES SPACED FROM EACH OTHER ON SAID FRAME TO PROVIDE A MAGNETIC FIELD FLUX BETWEEN A PAIR OF OPPOSED FACE THEREOF, A PLURALITY OF ARMATURE MEANS SPACED FROM EACH OTHER AND EACH MOUNTED FOR PIVOTAL MOVEMENT ON SAID FRAME WITHIN SAID FIELD FLUX AND IN A PLANE EXTENDING BETWEEN SAID OPPOSED FACES, A PLURALITY OF ARMATURE COIL MEANS EACH ASSOCIATED WITH ONE OF SAID ARMATURE MEANS, A PLURALITY OF PRINT WIRE MEANS EACH SECURED TO ONE OF SAID ARMATURE MEANS ON ONE SIDE OF ITS PIVOT AND SO MOUNTED FOR MOVEMENT THAT WHEN IT MOVES AT LEAST A PORTION THEREOF WILL MOVE IN A DIRECTION SUBSTANTIALLY NORMAL TO THE LONGITUDINAL AXIS OF THE ASSOCIATED ARMATURE MEANS, EACH SAID PRINT WIRE MEANS BEING MOUNTED FOR IMPACT ENGAGEMENT WITH A PRINT MEDIUM, CONTROL SIGNAL MEANS COUPLED TO SAID PLURALITY OF ARMATURE COIL MEANS AND SELECTIVELY OPERABLE TO PROVIDE CONTROL SIGNALS IN A FIRST DIRECTION THROUGH SELECTED ONES OF SAID ARMATURE COIL MEANS TO ROTATE THEIR ASSOCIATED ARMATURE MEANS IN A FIRST DIRECTION TO IMPEL THEIR PRINT WIRE MEANS TOWARD SAID PRINT MEDIUM AND AFTER A PREDETERMINED TIME INTERVAL TO PROVIDE CONTROL SIGNALS IN THE REVERSE DIRECTION THROUGH SAID SELECTED ONES OF SAID ARMATURE COIL MEANS TO ROTATE 